CHAPTER 30 – CURRENT ELECTRICITY Ohm’s law


Ohm’s law states that the current that flows through a conductor is directly proportional to its voltage.


This means that when resistance is fixed: • as the voltage increases, the current increases • as the voltage decreases, the current decreases


This relationship can be calculated using the following formula:


voltage = current × resistance V =


I × R


CONCENTRATE! The following triangle is a useful tool for remembering the formulae for calculating voltage, current and resistance. Simply cover the quantity you want to measure with your finger and use the formula that you can still see.


Eureka


Georg Ohm (1789–1854) was a German mathematician and physicist who investigated the relationship between the three main elements of electricity: voltage, current and resistance. Both Ohm’s law and the SI unit of resistance, the ohm, are named after him.


V I × R


: Fig. 30.21 Georg Ohm EXAMPLE


Question Calculate the voltage across a resistor of 200 Ω when the current flowing through it is 10 A. Solution V = I × R


V = 10 A × 200 Ω V = 2000 V


FIGURE IT OUT! 1. Calculate the voltage across a resistor of 50 Ω, if the current flowing through it is 5 A.


2. Calculate the resistance of a resistor when a current of 13 A is flowing through it and a voltage of 260 V is flowing across it.


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TEST TUBE 8. If a resistor has fixed resistance, what will happen to the current if voltage is decreased? Elements of Science 301


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